In a cellular communications network, base stations are distributed over a geographical region and provide areas of coverage typically referred to as macro cells. To gain access to a network, a mobile station, which may also be referred to as a user equipment (UE), attaches to a base station supporting the macro cell in which the UE is located. If signal conditions deteriorate, for example, because the UE moves to the edge of the macro cell where signal strength is lower, it may be necessary for the UE to access the network via a different base station offering a more satisfactory connection.
In a Universal Mobile Telecommunications System (UMTS) network, the UE measures the quality of signals received from other potential target macro cells to assist in deciding when handover may be necessary. A macro cell layer Radio Network Controller (RNC), which controls the base stations, makes the handover decision based on measurements taken by the UE. The mobile station however is only able to report pilot power measurements from potential target macro cells, that is the power of a so-called “primary spreading code”; it is not able to explicitly determine the cell identification of a measured signal. The actual target cell is then determined by the RNC making use of knowledge of an orthogonal assignment of these “primary spreading codes”. Following determination of the most likely handover target cell, a RELOCATION message is sent to the target cell via the RNC to initiate the handover procedures. This mechanism is in accordance with 3GPP standard TS 23.009 “Handover Procedures”.
Deployment of smaller, low power transceiver femto devices, or femto base stations, that support femto cells, provides alternative routes for access to a cellular communications network. A femto base station, typically for home or business use and with a small coverage area, is usually connected via a femto gateway to the core network of the cellular communications system via a backhaul internet connection. A macro cell of the cellular communications system may have overlapping coverage with multiple femto cells. FIG. 1 schematically shows a UMTS system having two cellular network base stations 1 and 2 with their associated macro cells, illustrated by broken lines, and having within their coverage areas a plurality of femto devices 3 to 9 with associated femto cells. The base stations 1 and 2 are connected to an RNC 10 which is connected to a Mobile Switching Center (MSC) 11. The MSC 11 is also connected via a femto gateway 12 to femto devices 3 to 9 (not all of these connections are explicitly shown in FIG. 1.)
When a UE, for example, UE 13 in FIG. 1 attached to a macro cell base station 2, takes measurements to seek a handover target, it may detect the femto cell 4 as a potential target. However, measurements taken by the UE are often insufficient to determine the identity of the most appropriate target femto cell for handover. There are only a limited number of primary spreading codes (only 512 codes are defined by 3GPP in the release 8 version of the standard) and it is probable that only ten to twenty of these codes would be available for all of the femto cells located within a particular macro cell, with the remaining codes used to identify neighboring macro and micro cells. Since it is likely that more than ten to twenty femto devices will be installed in each macro cell, it is unlikely that a unique identification can be made using the primary spreading codes. A mechanism is therefore required to determine which is the most likely target femto cell in order to be able to forward to it an incoming RELOCATION message from the macro cell RNC 10.
One potential solution involves multicasting an incoming RELOCATION message to all femto cells located within the macro cell coverage zone. However, this results in transmission of signaling messages to femto cells that are not suitable handover targets and is thus increases traffic without benefit. It may also unnecessarily overload the femto devices since each one in the macro cell would need to prepare itself for an incoming mobile station.